Influence of bed roughness in open channel

Proceeding of International Seminar on Application of Science Matehmatics 2011 (ISASM2011) PWTC, KL, Nov, 1-3, 2011

INFLUENCE OF BED ROUGHNESS IN OPEN CHANNEL

Zarina Md Ali1 and Nor Ashikin Sa ib2

1_{Department of Water and Environ mental Engineering, Universiti Tun Hussein Onn Malaysia}

86400 Parit Ra ja Batu Pahat, Johor MALA YSIA.

1

zarinaa@uthm.edu.my,

2

ashyk insaib@yahoo.com,

Roughness elements are essential parts of natural ecological system for most environmental standpoint. The variation of roughness coefficient, n values occur may be caused by channel properties, flow rate and others. The purpose of this study is to determine the affects of gravel bed to roughness characteristics in channel. An experimental works were carried out by using flu mes (10m x 0.3m x 0.46m) with gravel and smooth bed surface. The impact of both conditions on the rate of flow and roughness was determined with different set of slopes and flow rates. The roughness coefficient n obtained by flume gravel bed surface is bigger than flu me with smooth bed surface. The differences of flowrate, Qtheory and

Qexperiment ranged of 17 to 25%. As conclusion, flow rate and roughness coeffic ient

were influenced by bed roughness and slope; and categorized in subcritica l flow condition.

Ke ywor ds: Open channel flow; roughness; channel bed; gravel bed

Scope : Engineering

1. INTRODUCTION

Roughness elements are essential parts of natural ecological system for most environmental standpoint. The roughness characteristics of open channel are widely studied and published [1-3]. In open channel, Manning formula has been widely used to determine the roughness coefficient, n. The variation of roughness coefficient, n occurs due to the many contributing factors, i.e surface roughness, vegetation, channel irregularity, channel alignment, silting and scouring, obstruction and discharge [1]. The determination of n is become a challenge because the values cannot be computed equally for all types of open channels [4]. The influence of bed roughness gives some effect to flow rate and also the roughness characteristics [5]. The value of n expected will be lower when the bed material is smooth, and is relatively unaffected by changes in the depth of flow. However, for bed surface that

used of gravels or boulders, value of n is larger and may vary significantly with the

2.1 Sample material

Lightweight concrete which is formed by foam agent (protein-hydrolisation) plus with cement mixture was used in preparing the sample of blocks (400mm x 295mm). It is basically will reduce the load pressure during the experiment and easy to install in the

flume. A 40kg of sieve sand (size of 3mm) and 20kg cement were mixed by added

water as ratio of cement: sand : water is 1:2:0.5 respectively. While, ratio of foam:

water used is 1:10 respectively. Thus, 1800kg/m3 is a new density value that

computed by this mixture. Then, some blocks were prepared by added gravels with size of 20 – 30mm on the surface of concrete to construct gravel blocks. The blocks without gravel are known as smooth blocks.

2.2 Laboratory Works

The experiments were conducted in a rectangular flume with glass s ides with a flat bed. The size of flume is 10m long, 0.30m wide and 0.46m deep as in Figure 1. The flume was adjusted to slopes of 1:500 (0.0020), 1:400 (0.0025) and 1:300 (0.0033). Block samples; gravel and smooth concrete blocks were used as bed surface and placed along the flume to provide a uniform roughness so that no transition of depth when the flow encountered with gravel or plain concrete region. The experiments were carried out by using fixed flow rate; 0.020m3/s, 0.025m3/s, 0.030m3/s and

0.035m3/s by adjusting the water pump level. The depth of flow was measured at all

sections by measurement apparatus. At each section, four points of depths were to be taken with three points of velocity (current meter).

(a) Flu me with gravel bed surface (a) Flu me with smooth bed surface (Controller) Fig 1: Sche matic diagra m of laboratory e xperiment

3. Depth and Roughness Characteristics

3.1 Depth of Flow

Figure 2 shows the depths of flow in flume plotted based on fixed flow rates and

slopes which are 0.02m3/s, 0.025m3/s, 0.03m3/s and 0.035m3/s; and 1: 500, 1:400 and

Proceeding of International Seminar on Application of Science Matehmatics 2011 (ISASM2011) PWTC, KL, Nov, 1-3, 2011

bed surface (gravel and smooth bed surface) are fluctuated based on the flow rates and show a smallest different between flow rates and slopes; The average range of

depth on gravel bed is from 0.095m (Q=0.02m3/s, So=1/300) to 0.154m.

(Q=0.035m3/s, So=1/500) which is slightly higher than on smooth bed; from 0.08m

(Q=0.02m3/s, So=1/300) to 0.146m. (Q=0.035m

3

/s, So=1/500).

0.000 0.060 0.120 0.180

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in flume

D e p th , y 0.000 0.060 0.120 0.180

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d Sections in Flumes

D e p th , y

(a) 1:500 (b) 1:400

0.000 0.060 0.120 0.180

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in Flumes

D e p th ,y (m ) Legend: 0.000 0.060 0.120 0.180

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in flume

D e p th , y Q=0.02m3/s Q=0.025 Q=0.03m3/s Q=0.035m3/s (c) 1:300

(a). flows through gravel bed

0 0.06 0.12 0.18

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in Flumes

D e p th , y (m ) 0 0.06 0.12 0.18

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in Flume

D ep th , y (m )

(a) 1:500 (b) 1:400

0 0.06 0.12 0.18

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d Section in Flume

D e p th , y (m ) Legend: 0.000 0.060 0.120 0.180

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d

Sections in flum e

D e p th , y Q=0.02m3/s Q=0.025 Q=0.03m3/s Q=0.035m3/s (c) 1:300

(b). flows through smooth bed

A channel roughness can be described through Manning roughness coefficient n as stated at equation (1).

2 / 1 3 / 2

1

o

S

AR

Q

n

where:

Q = Fixed volume flow (m3/s), A= cross sectional area of flow (m2), n = Manning

coefficient of roughness,R = hydraulic radius (m), S = fixed bed slope.

Table 1 shows tabulated Froude nu calculated from average depth of each section and average velocity measured from experiment according to bed slopes and flow rates. The flow measurement on the gravel and smooth bed surface are in subcritical state with Froude number, Fr = 0.689 to 0.804 and Fr = 0.799 to 0.902 respectively. As comparison, Froude number produced by gravel concrete is slightly lower than produced by smooth concrete. However, Froude number increased when slope increases.

Table 1. Froude nu calculated from data taken on two different bed surfaces

Table 2 shows the roughness coefficient, n according to fixed flow rates and slopes. The average roughness coefficient, n yield from gravel bed is ranged of 0.009 to 0.0114 which is a slightly higher than smooth bed; range of 0.0085 to 0.0096. It means that material of bed surface; gravel gave a resistance force to the flow as stated in [1]. Furthermore, 20-30mm of gravel has more effect at steeper slopes as proven in [6] where 6 mm of gravel gave more influenced to slope of 1:100.

Slope Qexperiment

(m3/s)

Froude Nu

Gravel Smooth

1:500

0.020 0.689 0.799

0.025 0.670 0.799

0.030 0.715 0.782

0.035 0.712 0.775

1:400

0.020 0.722 0.813

0.025 0.721 0.821

0.030 0.735 0.818

0.035 0.728 0.822

1:300

0.020 0.755 0.888

0.025 0.714 0.879

0.030 0.804 0.902

Proceeding of International Seminar on Application of Science Matehmatics 2011 (ISASM2011) PWTC, KL, Nov, 1-3, 2011

Table 2. Average of roughness coefficient, n according to fixed flow rate and slopes

Slope Qexperiment (m

3 /s)

Average roughness coefficient, n

Gravel bed Smooth bed

1:500

0.020 0.0096 0.0088

0.025 0.0095 0.0086

0.030 0.0090 0.0086

0.035 0.0090 0.0086

1:400

0.020 0.0103 0.0096

0.025 0.0103 0.0094

0.030 0.0100 0.0092

0.035 0.0100 0.0091

1:300

0.020 0.0113 0.0089

0.025 0.0114 0.0088

0.030 0.0111 0.0085

0.035 0.0107 0.0086

Table 3. Average of computed flow rate, Qtheory and fixed flow rate, Qexperiment based

on slopes when flow through gravel and smooth bed surface

Slope

Flume with gravel bed Flume with smooth bed

Qexperiment (m3/s)

Qtheory (m3/s)

Qexperiment (m3/s)

Qtheory (m3/s)

1:500

0.020 0.024 0.020 0.021

0.025 0.028 0.025 0.026

0.030 0.036 0.030 0.032

0.035 0.039 0.035 0.036

1:400

0.020 0.025 0.020 0.021

0.025 0.030 0.025 0.025

0.030 0.034 0.030 0.032

0.035 0.040 0.035 0.036

1:300

0.020 0.025 0.020 0.020

0.025 0.030 0.025 0.026

0.030 0.036 0.030 0.031

0.035 0.041 0.035 0.037

Table 3 shows average of computed flow rate, Qtheory and to flow rate, Qexperiment based

on slopes when flow through gravel and smooth bed surface. The differences of

4. CONCLUSIONS

As conclusion, flowrate and roughness coefficient were influenced by bed roughness. Besides, influence of bed roughness has more effect at steeper slope, although the study was done in small-scale. The smooth bed surface also affects the differences between Qtheory and Qexperiment because of bed material. The variation of type of bed roughness shall be studied in future for further understanding.

Acknowledgement

The authors would like to thank the Min istry of Higher Education (Vot 0561), Malaysia and University Tun Hussein Onn Malaysia for sponsoring this research.

References

[1] Chow, V.T. Open-channel Hydraulics, Singapore. McGraw-Hill Book Co, 1973.

[2] Sturm, T. W. Open Channel Hydraulics; McGraw-Hill. UK, 2001.

[3] Jain, S. C. Open Channel Flow. John Wiley & Sons, 2001.

[4] Jarret, R.D., Determination of Roughness Coefficients for Streams in Colorado,

USGS Water Resources Investigation Report, 1985.

[5] McKeon, B.J. A model for “dynamic” roughness in turbulent channel flow.

Proceedings of the Summer Program, Center for Turbulence Research, 2008.

[6] Mohammed, M.Y., Al-Taee, A.Y. and Al-Talib, A.N., Gravel Roughness and